The plant pathogenic bacterium Pseudomonas syringae is noted for its diverse and host-specific interactions with plants (Hirano and Upper, 1990). A specific strain may be assigned to one of at least 40 pathovars based on its host range among different plant species and then further assigned to a race based on differential interactions among cultivars of the host. In host plants the bacteria typically grow to high population levels in leaf intercellular spaces and then produce necrotic lesions. In nonhost plants or in host plants with race-specific resistance, the bacteria elicit the hypersensitive response (HR), a rapid, defense-associated programmed death of plant cells in contact with the pathogen (Alfano and Collmer, 1997). The ability to produce either of these reactions in plants appears to be directed by hrp (HR and pathogenicity) and hrc (HR and conserved) genes that encode a type III protein secretion pathway and by avr (avirulence) and hop (Hrp-dependent outer protein) genes that encode effector proteins injected into plant cells by the pathway (Alfano and Collmer, 1997). These effectors may also betray the parasite to the HR-triggering R-gene surveillance system of potential hosts (hence the avr designation), and plant breeding for resistance based on such gene-for-gene (avr-R) interactions may produce complex combinations of races and differential cultivars (Keen, 1990). hrp/hrc genes are probably universal among necrosis-causing gram-negative plant pathogens, and they have been sequenced in P. syringae pv. syringae (Psy) 61, Erwinia amylovora Ea321, Xanthomonas campestris pv. vesicatoria (Xcv) 85-10, and Ralstonia solanacearum GMI1000 (Alfano and Collmer, 1997). Based on their distinct gene arrangements and regulatory components, the hrp/hrc gene clusters of these four bacteria can be divided into two groups: I (Pseudomonas and Erwinia) and II (Xanthomonas and Ralstonia). The discrepancy between the distribution of these groups and the phylogeny of the bacteria provides some evidence that hrp/hrc gene clusters have been horizontally acquired and, therefore, may represent pathogenicity islands (Pais) (Alfano and Collmer, 1997).
Pais have been defined as gene clusters that (i) include many virulence genes, (ii) are selectively present in pathogenic strains, (iii) have different G+C content compared to host bacteria DNA, (iv) occupy large chromosomal regions, (v) are often flanked by direct repeats, (vi) are bordered by tRNA genes and/or cryptic mobile genetic elements, and (vii) are unstable (Hacker et al., 1997). Some Pais have inserted into different genomic locations in the same species (Wieler et al., 1997). Others reveal a mosaic structure indicative of multiple horizontal acquisitions (Hensel et al., 1999). Genes encoding type III secretion systems are present in Pais in animal pathogenic Salmonella spp. and Pseudomonas aeruginosa and on large plasmids in Yersinia and Shigella spp. Genes encoding effectors secreted by the pathway in these organisms are commonly linked to the pathway genes (Hueck, 1998), although a noteworthy exception is sopE, which is carried by a temperate phage without apparent linkage to SPI1 in certain isolates of S. typhimurium (Mirold et al., 1999). Three avr/hop genes have already been shown to be linked to the hrp/hrc cluster in P. syringae: avrE and several other Hrp-regulated transcriptional units are linked to the hrpR border of the hrp cluster in P. syringae pv tomato (Pto) DC3000 (Lorang and Keen, 1995); avrPphE is adjacent to hrpY (hrpK) in Pseudomonas phaseolicola (Pph) 1302A (Mansfield et al., 1994); and hopPsyA (hrmA) is adjacent to hrpK in Psy 61 (Heu and Hutcheson, 1993). Other Pseudomonas avr genes are located elsewhere in the genome or on plasmids (Leach and White, 1996), including a plasmid-borne group of avr genes described as a Pai in Pph 1449B (Jackson et al., 1999).
Because Avr, Hop, Hrp, and Hrc proteins represent promising therapeutic treatments in both plants and animals, it would be desirable to identify other proteins encoded by the Pai's in pathogenic bacteria and identify uses for those proteins.
The present invention overcomes these deficiencies in the art.